This invention relates to improvements in automatic transmissions for vehicles.
In a vehicle provided with an automatic transmission in its power transmission system, the so-called "creeping" tending to gradually move the vehicle occurs frequently when the vehicle is brought to a halt with its engine under rotation. An automatic transmission designed for obviating such a danger is commonly known and is so constructed that a speed stage of a relatively small gear ratio (for example, a 3rd speed at which the gear ratio is 1.00) is attained and maintained in the transmission when the vehicle is brought to a halt and the engine is idling. However, in the known transmission of the construction above described, hydraulic fluid at a pressure (for example, 6 kg/cm.sup.2) which is as high as that supplied during driving is supplied to hydraulically-actuated friction engaging units engaged to attain the 3rd speed, in the halted state of the vehicle too. Therefore, when the hydraulic fluid is then discharged from the friction engaging units to attain the 1st speed of a larger gear ratio for starting the vehicle from its halted state, that is, when the hydraulic fluid is quickly discharged as shown by a broken curve A in FIG. 1 for attaining the 1st speed within a short period of time, transmission of torque at the 1st speed is started from a point B, that is, after a length of time t.sub.1 after the fluid discharge is started. Thus, the torque transmission at the 3rd speed is completely changed over to that at the 1st speed in a very short length of time t.sub.2. The known automatic transmission is therefore defective in that a very large shock is imparted during the shift for starting the vehicle. Further, when the hydraulic fluid is gradually discharged from the friction engaging units as shown by another broken curve C in FIG. 1 in order to prevent impartation of the shock due to such an abrupt shift, transmission of torque at the 1st speed is started from a point D, that is, after a length of time t.sub.3 after the fluid discharge is started. In such a case, the length of time of acceleration at the 3rd speed is extended, and the speed stage is shifted to the 1st speed under the condition of high engine torque. Therefore, the known automatic transmission is also defective in that a similarly large shock is imparted during the shift. Furthermore, when the automatic transmission of the construction above described is so designed as to attain the 1st speed by engagement of one of the hydraulically-actuated friction engaging units and a mechanical one-way clutch, reverse rotation of the engine is prevented by the action of the one-way clutch when the vehicle is standing still midway of a steel ascent, so that the vehicle can be maintained in the halted state. However, when the accelerator pedal is then lightly depressed to attain the 1st speed, the vehicle will move in the reverse direction or backward due to the shortage of transmitted torque. Therefore, the known automatic transmission is also defective in that the driver driving the vehicle will be unsettled, and a danger may be given rise to.